The present invention relates to an optical transmitter module and an optical communication system employing the same, specifically to a diode laser controlling module that has an etalon as the wavelength filtering device to stabilize the oscillating wavelength and an applied device and optical communication system using the same.
The optical fiber communication retains a high-speed large capacity over a long distance and a strong resistance against electromagnetic noises, which makes it possible to organize a communication system for assuring a high reliability. This system has been transmitting a light signal of one wavelength with one optical fiber. However, it has been requested to further expand the transmission capacity, accompanied with the recent incoming large-capacity information-oriented society. Accordingly, the technique on the wavelength division multiplexed optical communication system has been put to practical use, which transmits multiple light signals of different wavelengths with one optical fiber, and increases the number of communication channels to realize the large capacity. With regard to the wavelength of the light transmitted an optical fiber, the wavelength region of a low transmission loss in the optical fiber is used, and the 1.3 micron waveband and the 1.5 micron waveband are called the window of transmission. The wavelength breadths of these windows are limited. Wavelength spacing between adjacent channels has been narrowed as far as possible, whereby the number of transmission channels has been increased. At the present time, the frequency spacing is 200 GHz or 100 GHz; however, the tendency heads for 50 GHz, 25 GHz, and so forth. To convert them into the wavelength spacing, they are equivalent to about 1.6 nm, 0.8 nm, 0.4 nm, 0.2 nm, and 0.1 nm, respectively, which are being narrowed.
As the wavelength spacing becomes narrower as above, there arises a necessity to maintain the wavelengths of a laser source precisely uniformly. When the wavelength of a laser source wavers and reaches a wavelength of an adjacent channel, there appears crosstalk between adjacent channel signals on the receiver side, which cannot assure the reliability of information communications. This is the reason for the necessity. These wavelength (or, frequency) channels are named as the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) grid, which is widely recognized as the ITU recommendation (International Telecommunication Union Standard).
Against this backdrop, various methods have been proposed which control the wavelength of a laser source in the wavelength division multiplexed communication. For example, a newly devised method has been put forward which locks a wavelength by introducing the dielectric multi-layer film filter or the Fabry-Perot Etalon as the wavelength filtering device, and by feeding back the detected error to the operational temperature of a semiconductor laser. In particular, the etalon embraces the characteristics that transmission peaks appear repeatedly according to a multiple interference order. For this reason, it becomes possible to lock the wavelengths of plural wavelength channels with one wavelength filtering device by bringing the period of the transmission curve into coincidence with the ITU-T grid. For example, Japanese Patent Laid-open No. Hei 10-79723 discloses a method of dividing a light beam transmitted through the Etalon into two light beams, receiving both of the lights by a first and a second photo-detector, acquiring a wavelength error detection signal from a difference between both the received light signals, and locking the wavelength by using the wavelength error detection signal.